1. Field of the Invention
This invention is related to photochromic materials and is more particularly concerned with novel inorganic photochromic materials which are particularly useful for incorporation in ophthalmic quality plastic host materials.
2. Prior Art
Various types of photochromic compounds are known for use in optical and ophthalmic devices such as lenses, filters, and screens. These photochromic compounds change color on exposure to certain wavelengths of light or other electromagnetic radiation. These photochromic changes are reversible; that is, they change to and from the various colors on exposure to and withdrawal from the activating light radiation. It is known that the general basis of the photochromic reaction is produced by a photochromic substance which contains atoms or molecules capable of switching back and forth between and existing in two distinct energy states. These substances are induced into a higher energy state by absorption of the activating radiant energy which is generally of specific wavelengths defined by the particular materials, usually in the ultraviolet, and, in the absence of the activating radiant energy, return to their unactivated stable state. In the colored or activated state, they absorb certain ranges of light energy, and in the unactivated state the important photochromic materials pass most wavelengths of electromagnetic energy in the visible portion of the spectrum.
Photochromic materials are found in both plastics and inorganic glass materials. U.S. Patent application Ser. Nos. 180,221, filed Sept. 13, 1974; 370,247, filed June 15, 1973; and 445,029, filed Feb. 22, 1974, detail three specific photochromic systems which have been successfully incorporated into ophthalmic quality plastic host materials. However, one or more of a variety of difficulties is inherent with each of the foregoing applications (all of which applications are assigned to the Assignee of the present application). For example, many prior art photochromic systems which may be introduced into plastic materials, exist in the unactivated state as a colored dye. Upon activation with the incident radiation, they change to another colored state which passes less light within the visible spectrum. For some applications, such as ski goggles, such a condition is tolerable, and in fact, is even desirable. However, for normal usage, it is highly desirable to have a photochromic system which is essentially colorless in the unactivated state and which changes to an absorbing state with color upon activation. As will be understood, an absorbing state with color encompasses, for example, the shades of gray, blue, brown and green which are normally encountered in applications such as sunglasses. Some photochromic systems when introduced into plastic hosts, exhibit these color characteristics; however, all of those presently known which exhibit these characteristics are also characterized by having extremely short useful lives, such as a few days exposure to bright sunlight. That is, the dye system is susceptible to fatigue through a variety of mechanisms after a short period of exposure to sunlight. Therefore, these systems are not presently suitable for commercial applications requiring extended useful lives.
It is known to combine a photochromic material and a resinous material, for instance, U.S. Pat. No. 3,761,422 relates to the preparation of a photochromic plastisol composition containing metal complexes of diphenylthiocarbazone which can be formed into a film. U.S. Pat. No. 3,565,814 relates to a photochromic composition having a fast rate of color change comprising a polymer of lauryl methacrylate having dispersed throughout the body thereof various benzospiropyran compounds. U.S. Pat. No. 3,666,352 relates to a photochromic lens comprising a sheet of vinyl chloride-vinyl acetate copolymer containing a mercury dithiozonate compound laminated between glass or plastic layers.
The need for protection of photochromic materials against atmospheric oxygen has also been recognized by the prior art as a means of insuring reversibility, for instance, U.S. Pat. No. 3,716,489 discloses a method of producing a fast-acting photochromic filter in which the photochromic material is in the form of a solid solution in a solid optically transparent epoxy polymer or alternately a polycarbonate polymer matrix.
The desired characteristics for a photochromic material have been developed in various glasses. U.S. Pat. No. 3,208,860 to Armistead et al. is an example of such a photochromic glass. This patent teaches the formation of minute crystals of a silver halide within a silicate glass structure. The silver halide crystals are photochromic and exhibit an extended useful life.